The present invention relates to a method for removing 20 nutrient species such as ammonium and/or potassium and/or phosphate ions from wastewater which contains appreciable amounts of these species so that eutrophication and similar undesired effects related to the discharge of said species may be prevented.
Furthermore said nutrient species are recovered in form of the agronomically valuable slightly soluble hydrous salts MgNH.sub.4 PO.sub.4 and/or MgKPO.sub.4 by a precipitation technique, applied either directly to the wastewater or to the eluate solution obtained after said wastewater has been treated with suitable ion-exchange resins. According to the present invention, not only the serious pollution problems are associated to the discharge of nutrients prevented, but also one or more salts of great interest as fertilizer are recovered, the value of which may cover, at least partially, the cost of the treatment process.
Appreciable concentration of ammonium and/or potassium and/or phosphate ions may be often found in many wastewaters of industrial, civil or mixed origin, even though treated by conventional biological methods. Sometimes, high concentration of nutrients is found in secondary streams produced during the biological treatment process such as, for instance, in the anaerobic digester supernatant. The presence of these nutrient ions often prevents the discharge or the recycle of said wastewater. Ammonium and, more often, phosphates are usually responsible for well known eutrophication phenomena in the receiving water bodies, so that stringent limitations to the discharge of these ions have been introduced throughout the world. Furthermore, excessive amounts of potassium, as found for instance in wastewater of zootechnical origin or from the production of olive oil, may prevent the recovery as animal feeding of by-products from the purification of said wastewater.
Several methods are actually available to remove N and P species from wastewater. Biological nitrifraction -denitrification is proposed worldwide to convert NH.sub.4.sup.+ to N.sub.2. Other techniques for ammonia removal applied on a full scale basis, are breakpoint chlorination and atmospheric stripping. Phosphates are ordinarily post-precipitated with lime, alum, iron salts after the biological treatment of wastewater. In very few cases, simultaneous C and P biological removal during wastewater treatment has been reported. None of these methods, however, permits the recovery of the potential agronomic value of these compounds.
On the other hand, slightly soluble ("slow-release") hydrous MgNH.sub.4 PO.sub.4 and MgKPO.sub.4 salts, obtained in common industrial practice from the corresponding pure chemicals, are well-known highly valuable fertilizers, rated "premium quality" in the agronomic literature.
The present invention relates to a method for removing and recovering said ammonium and/or potassium and/or phosphate ions from wastewater which comprises:
permitting said wastewater to pass through at least one bed of ion exchange resin capable of removing selectively those nutrient ions up to the desired level
regenerating those ion-exchange resins with a suitable regenerant solution such as NaCl, so that said nutrient ions may be obtained in a much more concentrated form
adding to said regeneration eluates at least one Mg salt or a mixture thereof so that in proper pH condition hydrous MgNH.sub.4 P0.sub.4 and/or MgKP0.sub.4 slightly soluble salts, of great agronomic value, may be precipitated.
In the case that wastewater contains concentrated amounts of said nutrient species, the corresponding pre-concentration operation through ion exchange resin may be avoided and said nutrient ions may be precipitated directly from wastewater by addition of an Mg salt or mixtures thereof.
More often, an intermediate situation may occur, where the wastewater considered, or even secondary streams of it, contain different concentration of various nutrient ions. This is the case, for instance, of wastewater from pig factory, where concentrations up to 1000 ppm NH.sub.4 and 400 ppm K with only 10 ppm P occur; or, similarly, in the complete biological treatment (activated sludge+nitrification/denitrification) of municipal wastewater, where the final effluent still contains 3-10 ppm P and virtually no ammonium, but hundreds ppm of NH.sub.4 are still released in the supernatant solution from the anaerobic digestion of sludges.
In such cases the pre-concentration step through ion-exchange resins is applied only to the diluted nutrient ions, while the concentrated species may be precipitated directly as it is from the wastewater.
One may indicate respectively as .ltoreq.5 or .gtoreq.15 mmol/1 the concentration limit for each nutrient species at which pre-concentration through ion exchange is still required or not; in the intermediate range (5-15 mmol/1) proper decision should be checked individually.
Various types of ion-exchange resins may be used for selective removal of said nutrient ions depending on the nature of the wastewater considered.
NH.sub.4.sup.+ and K.sup.+ cations may be exchanged selectively by some zeolites, either natural (such as clinoptilolite, phillipsite, etc.) or synthetic. Porous, strongly basic organic anion-exchange resins, or even inorganic such as activated alumina, are proved to be useful for phosphate removal from wastewater.